Description: The specific aims of this Activity are: 1) to quantitatively assess whether didanosine (ddI) undergoes presystemic (first-pass) gut wall and/or liver metabolism after oral administration; 2) to quantitatively assess whether allopurinol (an inhibitor of xanthine oxidase) inhibits the metabolism of ddI; 3) to quantitatively assess, individually, the absorption kinetics of 3TC, indinavir, and ritonavir after single dose individual oral administration alone, and in the presence of co-ingested dietary lipid; 4) to quantitatively assess, individually, the effect of gastrointestinal (GI) motility (altered by administration of propantheline bromide, a potent GI motility inhibitor, and metoclopramide, a potent GI motility stimulator) on the in vivo absorption kinetics of 3TC, indinavir and ritonavir; 5) to quantitatively assess, individually, the effects of adsorbents present in over-the-counter antidiarrheal and antiemetic preparations on the in vitro binding and in vivo absorption kinetics of zidovudine (AZT), ddI, zalcitabine (ddC), stavudine (d4T), 3TC, indinavir, and ritonavir; 6) to quantitatively assess the pharmacokinetics and the penetration characteristics of select intravenously-administered anti-AIDs drugs into the central nervous system (CNS) using microdialysis; and 7) to develop and pharmacokinetically evaluate liposomal formulations of select anti-AIDS drugs for their targeted delivery to the macrophages and lymph tissue after intravenous drug administration.